Section 6: Junction Signals and Route Indicators

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As an alternative to the provision of separate signals for each route, a route indicator may be used in conjunction with a single signal. This is particularly advantageous where there is a great number of routes, such as on the approach to a large station. Route indicators were first used at London Bridge, in 1878.

The most basic type of route indicator displays a different character or characters depending on the route that is set. When the signal is at 'danger' no route indication is shown and the indicator is blank, except that an early design of route indicator displayed a white triangle of lights when not displaying a character [6.13]. When the signal was cleared, a route indication was exhibited [6.14].

The actual characters displayed by route indicators differ from place to place and drivers must learn their applications in each individual case. Accordingly, it is not possible to list every indication along with its meaning as these vary between signals in different locations. For example, the letter "B" can mean "Branch line" at one signal and "Bay platform" at a different signal. Any one route indication should, however, have the same application on all signals in the same locality. Certain indications are commonly assigned to specific applications. A number is frequently displayed to indicate a platform road (see [6.14]) and the letter "X" often refers to a 'wrong direction' move, e.g. towards a 'limit of shunt' indicator (see Section 5). Route indicators that display characters are now known generically as 'alphanumeric' route indicators. They have existed in a variety of forms including mechanical type [6.15], stencil type [6.16], projector type [6.17] and 'theatre' or 'multi-lamp' type [6.18]. Modern alphanumeric route indicators display similar indications using fibre-optic or LED technology. There are two sizes, known as 'standard' and 'miniature'. Standard indicators are usually associated with main signals, while miniature indicators are usually associated with shunting or subsidiary signals.

Alphanumeric route indicators are not suitable for high speed running. An alphanumeric route indicator may be provided on a high speed line if no indication is displayed for the high speed route, an indication being displayed only for a low speed divergence.

Directional shunting signals of the banner type were used in a few places, e.g. Glasgow St. Enoch (Glasgow & South Western Railway) in 1898. The banner in these signals could rotate either to the left [6.20] or right [6.21] when cleared, to indicate the direction of route.

The junction signalling practices applicable to semaphore signals were perpetuated in the earliest colour light signalling schemes. A colour light signal might be provided with an alphanumeric route indicator (see [6.17 & 6.18]); however, this was generally confined to lower speed complex areas such as large stations, as recommended by the report of the IRSE's Three-Position Signalling Committee published in 1924. The alternative was to provide multiple signal heads, one for each route [6.22]. The heads were stepped in height according to speed, the same as in semaphore signalling practice. Exceptionally, both methods of indicating the route could be implemented in combination on a signal with three or more routes, in which case one of a pair of signal heads would be provided with an associated route indicator.

The report of the IRSE's Three-Position Signalling Committee had also recommended that a colour light signal in rear of a junction signal should show a double yellow aspect (see [2.99]) whenever a diverging route was set. This idea found favour on the LMS, particularly following a derailment at Great Bridgeford on 17 June 1932; however, the practice was discontinued following an accident at Bourne End on 30 September 1945.

Another recommendation from the same report had suggested that where both routes were of a similar speed, the distant signal should be fitted with an illuminated indicator capable of displaying either "L" or "R" for the left-hand or right-hand route, respectively. This idea was not put into practice.

Practice regarding the colour light form of splitting distant signals varied between the different companies. LMS practice was to show only a single yellow aspect (see [2.97]) when the junction signal was at 'danger' (the junction signal may be another colour light or a semaphore). When the junction signal cleared, the splitting distant in rear could display a double yellow or green aspect alongside a single yellow aspect in a separate head [6.23]. Relative to this single yellow aspect, the other (less restrictive) aspect is positioned to the left or right, indicating which direction the train will take at the junction ahead. This form of signal was used only at junctions where both routes were of comparatively equal speeds.